Magnetic disc devices and optical disc devices have increased their speeds as well as capacities year by year, so that disc-driving spindle motors have been required not only to increase the speed and torque but also to decrease vibrations and noises.
For instance, a rewritable DVD sometimes undergoes a write at a high speed such as max. 6000 rpm, so that a slight vibration of the disc causes a miss. The spindle motor, which drives discs, is thus required in particular to reduce vibrations.
With respect to reducing vibrations of spindle motors (reducing the vibrations particularly in a radial direction), a hydrodynamic bearing is used mainly in fixed magnetic disc devices instead of a ball bearing conventionally used. The hydrodynamic bearing allows holding a shaft in a radial direction with a strong fluid-film in a non-contact state, so that vibrations can be reduced. This method has been widely accepted.
There is another method proposed, i.e. a bearing per se employs a sintered oilless bearing; however, its core shapes asymmetric, thereby producing a lateral pressure urging the rotor always in a radial direction. As a result, wobbles of the shaft are suppressed, and vibrations can be reduced.
A method employing the hydrodynamic bearing is disclosed in Japanese Patent Application Non-Examined Publication No. H09-285047, and a method employing the asymmetric core is disclosed in Japanese Patent Application Non-Examined Publication No. 2001-16806. The method of employing the hydrodynamic bearing needs to provide the bearing with extremely accurate machining, so that the bearing becomes costly and the motor tends to be expensive.
The latter method using the asymmetric core does not need special components at all, so that the cost can be lowered; however unexpected vibrations caused by the asymmetric core tend to occur, so that few products have been commercialized yet.